The present invention relates to a photoelectric conversion device. More specifically, the present invention relates to a photoelectric conversion device including a photoelectric conversion layer of semiconductor material that has a relatively small band gap and photosensitivity even in a longer wavelength region.
In a thin film-type photoelectric conversion device that uses a glass sheet as a substrate, a transparent conductive film, acting as a transparent electrode, is formed on the glass sheet, and a thin film photoelectric conversion unit including a photoelectric conversion layer is formed on the transparent conductive film. A tin oxide film often is employed as the transparent conductive film. The unevenness generated on the surface of the transparent conductive film with the growth of crystal grains has the effect of improving photoelectric conversion efficiency by trapping incident light in the photoelectric conversion layer or in the vicinity of the layer (i.e., light trapping effect). Thus, as for transparent conductive films, various surface shapes to improve the photoelectric conversion efficiency have been proposed.
Examples of the known thin film photoelectric conversion units are as follows: a unit including a photoelectric conversion layer formed of an amorphous silicon thin film, a unit including a photoelectric conversion layer formed of an amorphous silicon germanium thin film, and a unit including a photoelectric conversion layer formed of a crystalline silicon based thin film, such as microcrystalline silicon. Moreover, a tandem-type photoelectric conversion device has been under active development because of its ability to utilize light in a broad wavelength region. The tandem-type photoelectric conversion device includes two thin film photoelectric conversion units formed on a transparent conductive film, each unit having a different photoelectric conversion layer.
To increase the photoelectric conversion efficiency, a photoelectric conversion device requires some adaptation according to a photoelectric conversion layer to be used. For example, the thin film photoelectric conversion unit including the photoelectric conversion layer formed of a crystalline silicon based thin film (i.e., a crystalline silicon based thin film photoelectric conversion unit) has a smaller absorption coefficient than that of the amorphous silicon based unit. However, when the film thickness is increased simply to increase optical absorption, the manufacturing cost becomes higher. Therefore, the improvement in photoelectric conversion efficiency by taking advantage of the light trapping effect is important particularly to the photoelectric conversion device employing the crystalline silicon based thin film photoelectric conversion unit.
In general, the unit including the photoelectric conversion layer formed of an amorphous silicon germanium thin film or crystalline silicon based thin film has high sensitivity in the long wavelength region, compared with the unit including the photoelectric conversion layer formed of a general amorphous silicon thin film. Even with the amorphous silicon thin film, however, the spectral response becomes high in the long wavelength region as the film thickness increases. Therefore, when these thin films are used as a photoelectric conversion layer, it is necessary to pay considerable attention to the photoelectric conversion efficiency also in a relatively long wavelength region, e.g., at a wavelength of 650 nm or more.
However, a conventional photoelectric conversion device is not always provided with a structure suitable for the characteristics of its photoelectric conversion layer. For example, the photoelectric conversion device employing a crystalline silicon based thin film photoelectric conversion unit can have a large light trapping effect when the slope of concave and convex portions in the surface of a transparent conductive film is made sharp. However, this may degrade the quality of a crystalline silicon based thin film to be formed on the transparent conductive film. Even if the crystalline silicon based thin film is formed via other thin films, as in the tandem structure, the surface unevenness with a large degree of slope causes degradation in the crystallinity of the crystalline silicon. Thus, it is desired that the crystalline silicon based thin film-type photoelectric conversion unit should achieve the light trapping effect, which is important to improve the photoelectric conversion efficiency, without relying only on the surface unevenness of the transparent conductive film.
Even when using a photoelectric conversion layer that renders the photoelectric conversion efficiency in a relatively long wavelength region important, the conventional photoelectric conversion unit does not always adjust properly the characteristics of other members and thin films to be used with the photoelectric conversion layer, particularly their transmittance and contributions to the light trapping effect in that wavelength region.
Therefore, with the foregoing in mind, it is an object of the present invention to provide a photoelectric conversion device that includes a photoelectric conversion layer having high photoelectric conversion efficiency even in a relatively long wavelength region, i.e., a photoelectric conversion layer having a relatively small band gap, and that is provided with a structure capable of improving the photoelectric conversion efficiency of the photoelectric conversion layer. In particular, it is an object of the present invention to provide a photoelectric conversion device that improves the photoelectric conversion efficiency of a crystalline silicon based thin film photoelectric conversion unit without relying only on the light trapping effect obtained by a transparent conductive film.
To achieve the above objects, a photoelectric conversion device of the present invention includes the following: a transparent substrate having a first principal surface and a second principal surface that are parallel to each other; an antireflection film formed on the first principal surface; a transparent conductive film formed on the second principal surface; at least one photoelectric conversion unit formed on the transparent conductive film; and a back electrode formed on the photoelectric conversion unit. The antireflection film contains fine particles with a particle diameter in the range of 0.01 xcexcm to 1.0 xcexcm and has an uneven surface derived from the fine particles. The transparent substrate has a light transmittance of 75% or more in a wavelength region in the range of 800 nm to 900 nm when measured with the transparent conductive film formed thereon. At least one of the photoelectric conversion units includes a thin film of semiconductor material having a band gap of 1.85 eV or less as a photoelectric conversion layer.
It is preferable that the above photoelectric conversion device has photosensitivity even in a long wavelength region so that an external quantum efficiency at a wavelength of 700 nm is 0.2 or more. It is more preferable that the external quantum efficiency at the same wavelength is 0.3 or more. In the above photoelectric conversion device, when the thin film of semiconductor material is a crystalline silicon based thin film, it is preferable that the photoelectric conversion unit including the crystalline silicon based thin film as a photoelectric conversion layer has a thickness of 10 xcexcm or less.